For over a decade, the U.S. Department of Defense has been assembling the Global Positioning System (GPS), a constellation of 24 earth-orbiting satellites with altitudes of approximately 20,000 km and periods of about 12 hours. Each satellite transmits coded signals toward earth for reception by specially designed receivers. Given a complete constellation, the satellite orbits are constructed so that a receiver positioned anywhere on earth will be able to see at least four satellites. The initial motivation for the Global Positioning System was to provide military users with the capability of determining their positions and velocities and synchronizing their clocks. Since its inception, however, applications of GPS have become steadily broader in commercial and scientific fields, demanding accuracies in position and time far in excess of the original military requirements. Applications of GPS now involve high-accuracy measurements for geodesy, earth dynamics, ionosphere and troposphere investigations, clock synchronization, and orbit determination. The ever more stringent accuracy requirements of these applications have steadily increased the performance standards for GPS receivers.
To allow for ionosphere measurement and calibration, the GPS signal is transmitted at two RF frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. The L1 signal is a sum of two signals, the first comprised of a Gold code called the C/A (coarse/acquisition) code possessing a chip rate of 1.023 MHz and modulated on an L1 carrier and the second comprised of a pseudorandom code called the P (precise) code possessing a chip rate of 10.23 MHz and modulated on an L1 carrier in phase quadrature with the C/A carrier. The chip sequence of the slower C/A code repeats every millisecond and is used for acquiring the signal, particularly when the position and/or timekeeping of the receiver are relatively uncertain. For the faster P code, the chip sequence repeats every 7 days and is typically difficult to acquire without aiding from the C/A channel. The L2 signal consists of a carrier at 1227.6 MHz modulated by a P code that is currently identical to the P code on L1. For both the C/A and P codes, each satellite generates a unique code sequence, thereby allowing separation of signals. All three of these signals are further modulated by a common 50 Hz "telemetry" code, referred to as the data bits or navigation message. When interpreted, the data bits supply information about the health of the satellites, orbit parameters for each satellite, a clock offset for each satellite and other information.
Standard processing of GPS signals consists of correlating a received signal with a model signal constructed as a model carrier modulated by a model code. A number of techniques (e.g., maximizing correlation amplitude or minimizing tracking error) can be used to adjust the delay of the model code so that it is substantially aligned with the received code and to adjust the phase of the model carrier so that it is substantially locked to the received phase. Delay measured in this manner is a measure of the transit time (range) from the satellite to the receiver and is a sum of geometric, tropospheric, ionospheric and clock effects. It is sometimes referred to as pseudorange to denote the presence of the receiver clock error. In this disclosure, delay will also be referred to as group delay. Because of its higher chip rate, the P-code signal leads to more accurate measurements of group delay than the C/A-code signal. Phase extracted in such processing provides a measure of the same quantities as group delay but at much higher precision. However, phase usually only reveals time variations since it is afflicted with an unknown bias in the form of a phase ambiguity. In some applications, special processing can remove the phase ambiguity, thereby making phase a much more powerful observable. Delay values derived from phase will be referred to as phase delay.
At the discretion of the military, the P code can be encrypted by modulating the P code with another code, sometimes referred to as the A-code in the civilian sector. A discussion of the nature of the A code has been presented in U.S. Pat. No. 5,134,407 to Lorenz et al (1992) and will not be repeated here. From the civilian point of view, imposition of encryption has the effect of preventing unauthorized users from using the P-code signals in the standard fashion of correlating with a local model of the P code. Since phase and delay measurements derived from P-code signals are so valuable, various techniques have been devised, attaining various levels of performance, to process P-code signals without knowledge of the encryption code.
In this disclosure, the term "encryption-mode" will denote any mode of processing an encrypted P code signal without knowledge of the encryption code. The term "code mode" will denote a mode of processing an unencrypted P-code signal that correlates a replica of the known P code with the received signal and thereby despreads the signal spectrum. Since encryption does not affect the C/A channel, that channel is usually processed in a "code mode" with the known C/A code.
In some applications, signal fading due to multipath or scintillations is a problem, with fading occurring at different times for L1 and L2. If measurement of L1 and L2 amplitudes is a goal of the application, such as scintillation or atmospheric occultation measurements, the receiver should be capable of measuring L2 amplitude independently of L1 amplitude so that amplitude information is not compromised on the good channel when the other fades. Along related lines, if the application requires fast reacquisition of a faded channel, signal tracking for a given channel should be made independent of the other channel so that the good channel can maintain lock across the fading interval and then aid the weak channel to reacquire. Furthermore, if L2-P can be tracked independently of L1-P, the receiver can consist of only two channels, L1-C/A and L2-P. In applications for which it is feasible, such a two-channel receiver would lower cost and power consumption.
One trait common to all encryption-mode methods to date is lower SNR relative to code-mode operation, particularly at low elevations and/or at low antenna gain. When the P code is not encrypted, code-mode operation is greatly superior under those low signal conditions and therefore a very desirable option. Since it is not unlikely that the military will change its policy again and allow long intervals of unencrypted operation, it is therefore important to have a method that can function in either the code mode or the encryption mode so that code-mode operation can be selected when the P code is not encrypted. In such a dual-mode receiver, maximum commonality of hardware and software between modes would reduce cost and size. Thus, such commonality of operations between modes is an important consideration in judging encryption-mode methods.
As discussed below, some encryption-mode methods suffer from half-cycle ambiguities in L2-P phase. Such half-cycle ambiguities complicate processing to remove phase ambiguities and should be avoided.
Military policy changes have already unexpectedly taken place regarding planned times for encrypting the P code. Since GPS is primarily a military project, the goals and applications of the military will always take precedence over civilian applications, often in unanticipated and disruptive ways. Thus, it is important to make successful operation of receivers as immune as possible to policy changes by the military. One convention that potentially could be changed is the total commonality of the encryption code between L1 and L2. In principle, either the encryption-code signs or the encryption-code transition times could be made disparate between L1 and L2. Such a change would incapacitate some encryption-mode methods. Thus, a very desirable attribute of an encryption-mode method would be relative immunity to changes in the nature of the encryption code.
In summary, there is a need for a method that can operate in either the code mode (i.e., using P code when there is no encryption) or the encryption mode (when there is encryption) and satisfy all of the following criteria in either mode: exhibit strong SNR for P tracking; measure both delay and phase independently for all channels; avoid half-cycle ambiguities in L2-P carrier phase; have maximum commonality of hardware and software in the code mode and encryption mode; provide the optional capability of separately and independently tracking the L1 and L2 channels; make independent measurements of L1-P amplitude and L2-P amplitude; provide the option of operating with only the L1-C/A channel and L2-P channel eliminating the need for an L1-P channel; and not rely on the assumption the encryption code is the same for L1 and L2.
The following patents disclose methods for operating in the encryption mode, with the indicated advantages and disadvantages. In U.S. Pat. No. 4,797,677 to MacDoran and Spitzmesser (1989), an encryption-mode method, called delay-and-multiply, is proposed that, for either L1 or L2, delays the signal by 50 ns and multiplies it times the undelayed signal. The resulting product contains a periodic signal representing the P-code "clock" at 10.23 MHz plus a doppler effect. In another channel, the signal is squared, producing another periodic signal representing the carrier at twice the carrier frequency. The phase of each of these tones can be extracted to obtain measurements of carrier phase and P-code delay. The positive aspects of this method are: L2-P can be processed without L1-P processing, L1-P and L2-P amplitudes can be independently measured, and the method does not require the encryption code to be the same for L1 and L2. Unfavorable aspects are the following. Because of self-multiplication, this method suffers from greatly reduced SNR and from half-cycle ambiguities in carrier phase. Further, if a parallel code-mode system is implemented along with this method, there would be considerable disparity in code-mode and encryption-mode hardware and software. Thus, this invention falls short of the important criteria listed above.
In U.S. Pat. No. 4,463,357 to MacDoran (1984), an encryption-mode method is proposed that carries out at about a 20 MHz rate a straight cross-correlation of the L1 and L2 signals, thereby relying on the commonality of P code encryption between L1 and L2. In the MacDoran invention, the resulting correlation products lead to measurements of the differences in group delay between L1 and L2, which are used to extract the columnar electron content of the ionosphere between the satellite and the receiver. Although the MacDoran invention has the sole purpose of ionospheric measurement via differences in group delay and the MacDoran claims and disclosure do not foresee broader uses for the disclosed cross-correlation method, such a method can be extended to measure the difference in L1-P phase and L2-P phase, in addition to the difference in L1-P group delay and L2-P group delay. Given these observable differences, the broader method can use measurements of L1 phase and delay generated by the L1-C/A channel to extract phase and delay for L2-P. L2-P phase would have full-cycle ambiguities. L1-C/A phase and delay would serve as the L1 channel observables. No separate processing for L1-P is performed. The disadvantages of the extended cross-correlation method are as follows. Similar to squaring, the multiplication of L1 and L2 channels in cross-correlation causes a large loss of SNR relative to code-mode operation, leaving the cross-correlation SNR at far less than optimal levels at low elevation angles. Since L1-C/A phase and delay are used to recover L2-P phase and delay from the differences, the L2-P observables can be afflicted with errors from the L1-C/A channel. With regard to the commonality of operations between code mode and encryption mode, if a code-mode system were implemented along with cross-correlation, hardware could be nearly common but software would be disparate in many ways. This method assumes encryption is the same for L1 and L2, which currently is the case, but could be changed by the military. Amplitude for L2-P is not separately extracted but appears in a product of L1 and L2 amplitudes. Thus, this invention also falls short of the important criteria listed above.
In U.S. Pat. No. 4,972,431 to Keegan (1990), an encryption-mode method is proposed that improves upon the squaring method by reducing the signal-to-noise loss. In this method, a local model of the P code is correlated with the received encrypted L1 and/or L2 signals containing the product of the P code and the encryption code. The correlation removes the P code, leaving a pseudo-random sequence of A-code signs, each of duration of approximately 2 .mu.s. Since the A-code bandwidth is about 20 times smaller than the P code bandwidth, a 500-kHz filter can be used to improve SNR prior to squaring, leading to an SNR increase of about 13 dB. The advantages of this prior art method are: it can measure both phase and delay, separately measure L1-P and L2-P amplitudes, and process L2-P without L1-P. Its disadvantages are that it suffers from the aforementioned problem of half-cycle ambiguities as a result of the squaring operation and requires considerably different processing in the code mode and encryption mode, both in hardware and software. Thus, this invention also falls short of the important criteria listed above.
In U.S. Pat. No. 5,293,170 to Lorenz et al (1994), an encryption-mode method is disclosed that not only enhances the SNR of encryption-mode operation using a strategy similar to that of Keegan (see above), but also eliminates half-cycle ambiguities in carrier phase. In its first steps, the Lorenz method separately correlates the L1 signal and the L2 signal with a respective model of the P code and counter-rotates with a respective model carrier. The resulting correlation products are then summed and dumped over successive A chips to increase SNR. For both the L1 and L2 signals, the sign of prompt in-phase sum is then extracted as an estimate of the A-code sign. To reduce the effect of the sign flips of the A code, the A-code sign from L1 is multiplied times all correlation products from L2 and, in mirror image fashion, the A-code sign from L2 is multiplied times all correlation products from L1. Subsequent processing carries out the usual functions of tracking delay and phase, generating feedback, and measuring phase and delay. In this method, SNR is increased by about 13 dB relative to squaring and the half-cycle ambiguities in phase are eliminated through a stepwise process based ultimately on L1-C/A phase. The advantages of this method are: the SNR relative to straight squaring is substantially increased, half-cycle ambiguities are eliminated, both delay and phase are measured for L1-P and L2-P, and commonality of hardware and software between code mode and encryption mode is nearly total, except for the indicated A-code-sign operations between L1 and L2. However, the Lorenz invention, which comes closest to the criteria set forth above, does not meet all of the criteria and has the following important disadvantages relative to the present invention:
1. In the Lorenz invention, P-channel processing does not produce separate measurements of L1 and L2 amplitudes but provides measurement of products of the two amplitudes. L-band-specific amplitude measurements are important in some applications, such as investigations of ionospheric scintillation and atmospheric occultation for which wide variations in amplitude occur. In the Lorenz invention, if the amplitude for L1 moves to a value too small to measure, for example, the amplitude of L2 can not be measured. If the L1 amplitude decreases to a small value, the error in the deduced L2 amplitude will be large. In contrast, the present invention can make separate and independent measurements of either the L1 or L2 amplitude, regardless of the value of the other amplitude. PA1 2. Related to the preceding disadvantage is the undesirable feature of the Lorenz invention that lock on the L2-P signal is lost if the lock on L1-P is lost, which means signal contact can not be maintained. Thus, even if amplitude measurement is not a goal, interdependence of channels is a detriment. In contrast, with the present invention, it is possible to construct a receiver in which lock can be maintained independently on L1 and L2 and either channel can maintain contact with the signal if the other is lost. Such a capability is important in applications where L-band-specific signal fading, such as that due to scintillations or multipath, is a problem and where fast reacquisition of the faded channel is important. PA1 3. When L1-P and L2-P phases and delays are measured as described in the Lorenz invention and in the present invention, the resulting precision in dual-band-calibrated output, typically the output of most importance in high-accuracy surveying, geodesy and orbit determination, is better by approximately 20% in phase delay and 30% in group delay for the present invention than for the Lorenz invention, given the nominal GPS power ratio for L1 and L2. PA1 4. The Lorenz invention can not be operated without the L1-P channel. In contrast, in applications that allow a half-cycle ambiguity in L2 phase, the present invention can be implemented with only two channels (L1-C/A, L2-P) rather than three (L1-C/A, L1-P, L2-P) and still provide valuable dual-band performance. This 33% reduction in the number of channels would offer the advantages of lower power, lower gate count, and lower cost. PA1 5. The Lorenz invention is based on the assumption that the A-code signs for L1-P and L2-P are the same, which means the Lorenz invention would not function if the military changed this current convention. In contrast, since mathematics dictates that the A-code will always be the same on the two quadrature components of a given RF channel, the present invention is immune to such changes and would still operate at full capability if this convention were changed. PA1 6. The Lorenz invention assumes the transition times of the L1-P A-code are substantially coincident with those of the L2-P A-code. If the military changed this convention, the Lorenz invention would be compromised. In contrast, following the argument of the preceding paragraph, the present invention is immune to such transition-time changes. By similar argument, the present invention does not have to contend with possible time dealignment of the L1-P and L2-P signals incurred in route to the receiver, such as that caused by the ionosphere.
This review of known patents on encryption-mode processing shows none of the reviewed methods satisfies all of the important criteria summarized above. As shown below, the current invention does meet all of these criteria.